1. Field of the Invention
The present invention relates to the techniques for orthogonal frequency division multiplexing (OFDM), and more particularly to apparatus and method for determining positions of pilot sub-carriers in an OFDM symbol.
2. Description of Related Art
OFDM (orthogonal frequency division multiplexing) modulation techniques are widely used in communication areas such as 802.11, DAB, DVB, DSL communication protocol standards due to high channel efficiency and good anti-fading capability therein. In an OFDM communication system, to ensure orthogonality between sub-carriers is one of key factors to receive data exactly and requires frequency synchronization accurately. For receiving data exactly by a receiver, it also requires to perform a channel estimation based on pilots. Before the channel estimation can be performed, frame synchronization needs to be accomplished beforehand. All processes mentioned above need to determine positions of the pilot sub-carriers in an OFDM symbol first.
The OFDM communication system transmits data signals and pilot signal via a plurality of sub-carriers. The pilot sub-carriers are distributed periodically in time domain and in frequency domain and the pilot signal has special and known amplitude and phase. Therefore, frame synchronization and frequency synchronization can be accomplished by determining positions of the pilot sub-carriers in the OFDM symbol. It should be noted that fractional frequency offset estimation and correction are required to be accomplished beforehand in order to ensure orthogonality between the sub-carriers in the OFDM symbol, which is already introduced in many reference documents.
There are two conventional methods to determine positions of the pilot sub-carriers in the OFDM symbol. The first method is to determine positions of the pilot sub-carriers in one OFDM symbol period by utilizing the periodic distribution of the pilot sub-carriers in time and frequency domains and characteristics of energy of the pilot sub-carriers higher than that of data sub-carriers. The first method has one advantage of being short in time and one disadvantage of inaccuracy in seriously multipath fading and lower signal noise ratio conditions which may cause energy difference between the pilot sub-carriers and the data sub-carriers to be insignificant. The second method is to determine positions of the pilot sub-carriers by utilizing periodic distribution of the pilot sub-carriers in time and frequency domains and characteristics of correlation between the strong pilot signals being stronger and characteristics of correlation between the data signals being smaller. The second method may be referred to in a Chinese patent application No. CN200410092778.6, entitled “a method for integral frequency synchronization in OFDM”, which is hereby incorporated by reference. Although the second method has higher resolution performance, it needs at least two OFDM symbol periods and is sensitive to errors caused by clock frequency offsets. The clock frequency offsets between clock frequencies of a receiver and a transmitter are often inevitable and may introduce different phase rotations at different frequencies, thereby decreasing or and even canceling the effect of coherent superposition in a special clock frequency offset so that the wrong decision may be made.
Thus, there is a need for improved techniques for determining positions of pilot sub-carriers in an OFDM symbol.